This example shows a custom method metaclass that models public versus private methods. If a method is defined as private, it adds a wrapper around the method which dies unless it is called from the class where it was defined.

The way the method is added to the class is rather ugly. If we wanted to make this a real feature, we'd probably want to add some sort of sugar to allow us to declare private methods, but that is beyond the scope of this recipe. See the Extending recipes for more on this topic.

The core of our custom class is the policy attribute, and _add_policy_wrapper method.

You'll note that we have to explicitly set the policy attribute in our constructor:

$self->{_policy} = $options{policy};

That is necessary because Moose metaclasses do not use the meta API to create objects. Most Moose classes have a custom "inlined" constructor for speed.

In this particular case, our parent class's constructor is the wrap method. We call that to build our object, but it does not include subclass-specific attributes.

The _add_policy_wrapper method is where the real work is done. If the method is private, we construct a wrapper around the real subroutine which checks that the caller matches the package in which the subroutine was created.

If they don't match, it dies. If they do match, the real method is called. We use goto so that the wrapper does not show up in the call stack.

Finally, we replace the value of $self->{body}. This is another case where we have to do something a bit gross because Moose does not use Moose for its own implementation.

When we pass this method object to the metaclass's add_method method, it will take the method body and make it available in the class.

Finally, when we retrieve these methods via the introspection API, we can call the is_public and is_private methods on them to get more information about the method.

A custom method metaclass lets us add both behavior and meta-information to methods. Unfortunately, because the Perl interpreter does not provide easy hooks into method declaration, the API we have for adding these methods is not very pretty.

That can be improved with custom Moose-like sugar, or even by using a tool like Devel::Declare to create full-blown new keywords in Perl.